Al-zn alloy for die casting having both high strength and high thermal conductivity

ABSTRACT

The present invention relates to an aluminum alloy which has excellent thermal conductivity as well as high castability and satisfactory tensile characteristics, and is thus applicable to various kinds of structural products that require heat dissipation characteristics. According to the present invention, the aluminum alloy for die casting includes 5.5-8.5 wt % of zinc (Zn), 0.5-3.0 wt % of magnesium (Mg), 0.45-1.5 wt % of iron (Fe), and the remainder consisting of aluminum (Al) and inevitable impurities.

TECHNICAL FIELD

The present invention relates to an aluminum alloy for die casting having high strength and thermal conductivity, and more particularly, to an aluminum alloy having high tensile strength and thermal conductivity as well as high castability.

BACKGROUND ART

Die casting is a precision casting process in which a molten metal is injected into a steel mold which is precisely machined so as to perfectly match a cast shape to obtain a casting having the same shape as the mold. Products through this casting process are referred to as die-cast molded articles.

Die-cast molded articles have accurate dimensions, and are thus characterized by having excellent mechanical properties and the possibility of mass production in addition to the advantage that there is almost no need to perform finishing. Examples of metals used for die casting generally include alloys containing zinc, aluminum, tin, copper, magnesium, or the like. These alloys are melted into molten metal and then injected into a mold of a die casting machine via a pressure device using pneumatic, hydraulic, oil pressure, or the like, and quenched to be solidified.

Die-cast molded articles manufactured through these processes are being used in various fields. In particular, they are largely applied to automotive components, and also extensively used in the manufacturing of components, such as components of electric devices, optical devices, vehicles, weaving machinery, construction equipment and measuring instruments.

Meanwhile, Al—Si based alloys and Al—Mg based alloys which have high castability are mainly used as aluminum alloys for die casting. However, Al—Si based alloys or Al—Mg based alloys have a tensile strength of 230 MPa or more, but a low thermal conductivity of 90 to 130 W/mK. Therefore, the use thereof in heat dissipating components for electrical, electronic, and automotive applications is limited, which require high tensile strength and thermal conductivity, such as a thermal conductivity of 130 W/mK or more as well as a tensile strength of 230 MPa or more.

Also, while in heat dissipating components requiring a high thermal conductivity, some die-cast molded articles made of pure aluminum having a very high thermal conductivity of 220 W/mK or more are used in rotors for electrical and electronic devices, pure aluminum has an excellent thermal conductivity, but a low tensile strength of about 100 MPa. Therefore, the application of pure aluminum to structural components requiring a high tensile strength as well as a high thermal conductivity is limited.

Accordingly, the development of aluminum alloys for die casting having a high thermal conductivity of 130 W/mK or more and a high tensile strength of 230 MPa or more as well as high castability is urgently required to use such aluminum alloys in heat dissipating components or the like for electrical, electronic, and automotive applications. However, aluminum alloys for die casting having a high thermal conductivity of 130 W/mK or more and a high tensile strength of 230 MPa or more as well as high castability have not yet been developed, and thus, Al—Si based alloys, Al—Mg based alloys, and the like which have a tensile strength of 230 MPa or more and a thermal conductivity of 90 to 130 W/mK are currently being used as aluminum alloys for die casting in heat dissipating components for electrical, electronic, and automotive applications.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention is devised to solve the above-described problems, and the purpose of the present invention is to provide an aluminum alloy for die casting including zinc as a main alloying element with magnesium and iron content being controlled, and having a thermal conductivity of 130 W/mK or more and a tensile strength of 230 MPa or more as well as high castability.

Technical Solution

An embodiment of the present invention provides an aluminum alloy for die casting including 5.5-8.5 wt % of zinc (Zn), 0.5-3.0 wt % of magnesium (Mg), 0.45-1.5 wt % of iron (Fe), and the remainder consisting of aluminum (Al) and inevitable impurities.

In some embodiments, the aluminum alloy may have a thermal conductivity of 130 W/mK and also have a tensile strength of 230 MPa or more.

In other embodiments, ΔT may be 70° C. or less, wherein ΔT is a difference (T_(L)−T_(S)) between liquidus temperature (I_(L)) and solidus temperature (T_(S)) and is used as an indicator of castability.

In still other embodiments, the content of zinc may be 5.5 to 8.0 wt % to further improve thermal conductivity and strength.

In even other embodiments, the content of magnesium may be 0.7 to 2.0 wt % to further improve thermal conductivity and strength.

In yet other embodiments, the content of iron may be 0.5 to 1.1 wt % to further improve thermal conductivity and strength.

Advantageous Effects

An aluminum alloy according to the present invention includes zinc as a main alloying element with the compositions of magnesium and iron being controlled, and thus secures sufficient castability that is required to obtain sound molded articles in a die casting process, and also has a thermal conductivity in the range of 130 to 165 W/mK and a tensile strength in the range of 230 to 300 MPa, so that the aluminum alloy may be appropriately used in manufacturing heat dissipating components for electrical, electronic, and automotive applications requiring a high thermal conductivity and a considerable level of mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for evaluating castability of aluminum alloys.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, aluminum alloys according to preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. Therefore, it will be apparent to those of ordinary skill in the art that various modifications can be made to the present invention without departing from the scope and spirit of the present invention.

Also, terms in the singular form used for describing embodiments of the present invention may include the plural form unless otherwise stated.

An aluminum alloy according to the present invention is an aluminum alloy for die casting having a high thermal conductivity and including zinc, magnesium and iron as alloying elements, wherein the aluminum alloy includes 5.5-8.5 wt % of zinc (Zn), 0.5-3.0 wt % of magnesium (Mg), 0.45-1.5 wt % of iron (Fe), and the remainder consisting of aluminum (Al) and inevitable impurities.

The present invention provides an aluminum alloy for die casting having a high strength and a high thermal conductivity, such as a thermal conductivity of 130 W/mK or more and a tensile strength of 230 MPa or more, as well as high castability, wherein the aluminum alloy includes, in combination, alloying elements capable of improving castability of aluminum, alloying elements capable of affecting solid solution strengthening and precipitation strengthening when dissolved in an aluminum-based metal, and alloying elements capable of minimizing a reduction in thermal conductivity due to very low solid solubility in the aluminum-based metal and increasing tensile strength due to precipitation as intermetallic compounds as well as preventing die soldering when aluminum alloy products are molded by die casting, according to the respective compositions.

The reasons for the addition of each of the above alloying elements and for limiting the content are as follows.

Zinc is an alloying element which may be added in aluminum to increase tensile strength through a solid solution strengthening effect, and also increase tensile strength while minimizing a reduction in the thermal conductivity of aluminum due to precipitation as intermetallic compounds such as MgZn₂. It is preferable that the alloy according to the present invention includes 5.5 to 8.5 wt % of zinc. When the content of zinc is less than 5.5 wt %, a tensile strength of 230 MPa or more may not be obtained. On the other hand, when the content of zinc is more than 8.5 wt %, the thermal conductivity of the alloy is lowered such that a thermal conductivity of 130 W/mK or more may not be obtained. It is more preferable that the content of zinc is 5.5 to 8.0 wt % because thermal conductivity and strength of an aluminum alloy may be further improved.

Magnesium is an alloying element which may be added in aluminum to improve castability of aluminum alloys, and also increase tensile strength while minimizing a reduction in thermal conductivity due to precipitation as intermetallic compounds such as MgZn₂. It is preferable that the aluminum alloy for die casting according to the present invention includes 0.5 to 3.0 wt % of magnesium. When the content of magnesium is less than 0.5 wt %, a tensile strength of 230 MPa or more may not be obtained. On the other hand, when the content of magnesium is more than 3.0 wt %, the thermal conductivity of the alloy is lowered such that the thermal conductivity of 130 W/mK or more may not be obtained. It is preferable that the content of magnesium is 0.7 to 2.0 wt % because the thermal conductivity and strength of an aluminum alloy may be further improved.

Iron has a very low solid solubility of 0.052 wt % in aluminum at room temperature and is mostly crystallized as an intermetallic compound such as Al₃Fe after casting. Thus, iron is an alloying element which may be added in aluminum to increase strength while minimizing a reduction in the thermal conductivity of aluminum, and also prevent die soldering when aluminum alloy products are molded by die casting. It is preferable that the alloy for die casting according to the present invention includes 0.45 to 1.5 wt % of iron. When the content of iron is less than 0.45 wt %, the effect of preventing die soldering is lowered. Thus, the soldering of products may occur in some portions of a mold when the products are molded by die casting, and the tensile strength may not be sufficient. On the other hand, when the content of iron is more than 1.5 wt %, an excessive amount of an Fe-rich phase is crystallized in the alloy such that castability of the alloy may be deteriorated. It is preferable that the content of iron is 0.5 to 1.1 wt % because the thermal conductivity and strength of an aluminum alloy may be further improved.

Inevitable impurities mean impurities which are inadvertently added to alloys from raw materials or manufacturing equipment in a process of manufacturing the alloys according to the present invention. Examples of impurities include Ti, Cr, V, Mn, Li, and Zr, and each element of the impurities is maintained in an amount of 0.1 wt % or less, and preferably 0.01% or less so that the properties of the alloys are not affected by the impurities.

EXAMPLE

Aluminum alloys for die casting having a high strength and a high thermal conductivity according to embodiments of the present invention will be described in detail with reference to Tables 1 and 2 below.

The present inventors prepared alloy specimens having compositions shown in Table 1 below by using a melt stirring method which is typically used in die casting, in order to prepare an aluminum alloy for die casting having a high strength and a high thermal conductivity.

TABLE 1 Composition (wt %) Alloy Zn Mg Fe Si Al Example 1 6.06 0.64 1.02 — bal. 2 6.10 1.25 1.04 — bal. 3 6.05 1.59 0.53 — bal. 4 5.92 2.08 0.60 — bal. 5 7.93 0.73 0.93 — bal. 6 8.14 1.62 0.97 — bal. 7 7.90 2.07 0.49 — bal. 8 8.10 2.71 0.56 — bal. Comparative 9 0.92 0.21 0.97 10.20 bal. Example 10 1.95 1.11 0.52 — bal. 11 2.30 1.11 0.53 — bal. 12 9.16 2.22 0.29 — bal. 13 3.97 0.39 0.96 — bal.

Specifically, the raw materials of the aluminum alloy were prepared to have the compositions shown in Table 1, the raw materials were then charged into an electric resistance melting furnace and melted to prepare molten metals in the atmosphere, and then fluidity test specimens for evaluating castability were prepared by using a mold and also specimens for evaluating properties were prepared, which were used to measure thermal conductivities, liquidus temperatures, solidus temperatures, and the like.

With respect to favorable thermal conductivity, which is one of main objectives of an alloy according to embodiments of the present invention, first, the electrical conductivities of prepared specimens were measured by using an electrical conductivity meter at room temperature, and then the thermal conductivities were obtained through a conversion using [Formula 1].

K=5.02σT×10⁻⁹+0.03  [Formula 1]

where, K is a thermal conductivity, σ is an electrical conductivity, and T is an absolute temperature.

Also, castability evaluation, which is essential in die casting, was performed by flow length evaluation according to a generally known method in the art and by measuring the difference between liquidus temperature (T_(L)) and solidus temperature (T_(S)) using a thermal analyzer. In the method for evaluating flow length, the molten alloy was injected into a fluidity test mold maintained at a temperature of 200° C. as shown in FIG. 1, the solidified length was measured after the molten alloy flowed to some distance, and the flow length test specimen in FIG. 1 had a width of 12 mm, a thickness of 5 mm and a maximum length of 780 mm.

Table 2 below shows evaluation results of the flow length, the thermal conductivity, the liquidus temperature (T_(L)), the solidus temperature (T_(S)), the difference between the temperatures (ΔT=T_(L)−T_(S)), and the tensile property for each alloy.

TABLE 2 Flow Thermal Liquidus Solidus Tensile Yield length conductivity temp. temp. ΔT strength strength Elongation Alloy (mm) (W/mK) (° C.) (° C.) (° C.) (MPa) (MPa) (%) Example 1 780 163 649 616 33 254 210 12 2 720 152 646 600 46 307 250 8 3 780 152 640 587 53 280 242 4 4 780 148 640 577 63 256 190 8 5 720 156 638 600 38 293 231 6 6 770 144 643 589 54 285 216 4 7 780 144 639 572 67 242 202 2 8 780 133 637 567 70 290 185 2 Comparative 9 780 95 575 501 74 134 120 3 Example 10 — 175 — — — 141 103 30 11 — 171 — — — 151 110 22 12 — 116 — — — 299 230 2 13 — 175 — — — 171 122 20

As identified in Table 2 above, all of the aluminum alloys in Examples 1 to 8 according to the present invention have a thermal conductivity of 130 W/mK or more, which is equal to or higher than the level required in various heat dissipating components.

Also, the flow length and the ΔT shown in Table 2 are primary indices capable of evaluating the castability of alloys, in which a higher flow length means better fluidity of an alloy, and a smaller ΔT means better castability, wherein ΔT is the difference (T_(L)−T_(S)) between liquidus temperature and solidus temperature

As identified in Table 2 above, it can be seen that the aluminum alloys in the examples according to the present invention have a flow length corresponding to 90% or more of 780 mm which is the flow length of Al—Si alloy (ADC12, Comparative Example 9) widely used as an aluminum alloy for die casting.

Also, as identified in Table 2 above, ΔT of the aluminum alloys in the examples according to the present invention are 33 to 70° C., which are lower as compared with 74° C. which is ΔT of the alloy in Comparative Example 9, wherein ΔT is the difference (T_(L)−T_(S)) between liquidus temperature and solidus temperature.

In other words, aluminum alloys according to embodiments of the present invention have castability equal to or more excellent than that of a typical Al—Si alloy widely used as an aluminum alloy for die casting.

Furthermore, as identified in Table 2 above, the alloys in Examples 1 to 8 according to the present invention have a tensile strength of 240 MPa or more, which is higher than that of a typical Al—Si Alloy (ADC 12, Comparative Example 9) widely used as an aluminum alloy for die casting.

The alloys in Comparative Examples 10 and 11 have a zinc content of 1.95 wt % and 2.31 wt %, respectively, which are lower than those of the alloys in the examples according to the present invention, and as a result, they have a thermal conductivity higher than 170 W/mK, but tensile strengths of 141 MPa and 151 MPa, respectively, which are lower than those of the alloys in the examples according to the present invention.

Furthermore, the alloy in Comparative Examples 12 has a zinc content of 9.16 wt %, which is higher than those of the alloys in the examples according to the present invention, and as a result, it has a thermal conductivity of 116 W/mK, which is relatively lower than those of the alloys in the examples according to the present invention. Thus, it is difficult to use in components requiring highly efficient heat dissipation characteristics.

Furthermore, the alloy in Comparative Example 13 has a zinc content of 3.97 wt % and a magnesium content of 0.39 wt %, which are lower than those of the alloys in the examples according to the present invention, and as a result, it has a relatively higher thermal conductivity of 175 W/mK, but a relatively lower tensile strength of 171 MPa than the alloys in the examples according to the present invention, respectively.

As described above, the aluminum alloys according to embodiments of the present invention may be appropriately used as an aluminum material for die casting heat dissipating components requiring a certain level of tensile strength and excellent thermal conductive properties required for components as well as a castability sufficient to perform die casting. 

1. An aluminum alloy for die casting comprising 5.5-8.5 wt % of zinc (Zn), 0.5-3.0 wt % of magnesium (Mg), 0.45-1.5 wt % of iron (Fe), and the remainder consisting of aluminum (Al) and inevitable impurities.
 2. The aluminum alloy of claim 1, wherein the content of zinc is 5.5 to 8.0 wt %.
 3. The aluminum alloy of claim 1, wherein the content of magnesium is 0.7 to 2.0 wt %.
 4. The aluminum alloy of claim 1, wherein the content of iron is 0.5 to 1.1 wt %.
 5. The aluminum alloy of claim 1, having a thermal conductivity of 130 W/mK or more and a tensile strength of 230 MPa or more.
 6. The aluminum alloy of claim 1, wherein a difference between a liquidus temperature (T_(L)) and a solidus temperature (T_(S)) of the aluminum alloy is 70° C. or less. 